Product packaging has been changing from sealed metal cans and glass bottles to sealed flexible packages such as pouches. As one example tuna fish is now available in both traditional metal cans and flexible pouches. The flexible package when filled with a food or other product and closed or sealed can be readily changed in shape. The flexible package is typically prepared from two layers of flexible packaging material that are overlaid and sealed around most of their periphery to form a cavity inside. Typically the two layers of flexible packaging material are heat sealed by applying heat and pressure to fuse the layers together around a thin portion of the package periphery. Food or other product is placed in the cavity through an opening and the opening is closed by heat sealing the layers together. The sealed package and enclosed product can be heated for preservation purposes. In some demanding applications the sealed package and enclosed product can be boiled in water at 100° C.
Flexible packaging material is prepared by laminating two or more layers of film. Each film is chosen for specific properties. For example, a flexible packaging material can be a lamination of three layers. The inner layer will contact the packaged product. Polypropylene has desirable product contact properties as well as heat sealability and can be used as an inner layer. The middle layer will provide a barrier to moisture, oxygen and/or light. Metal films or foils have desirable barrier properties and metal films such as aluminum foil can be used as a middle layer. The outer layer will provide protection for the package and also provides a surface for printing information such as contents, packaging date, warnings, etc. Polyester films are tough, can receive printing ink and can be used as an outer layer. Flexible packaging material can range in thickness from about 13 to about 75 micrometers (0.0005 inches to 0.003 inches).
Each layer of the flexible packaging material is bonded to the adjacent layer by an adhesive. Adhesive can be applied to the layer from a solution in a suitable solvent using gravure or smooth roll coating cylinders or from a solvent-free state using special application machinery and that layer is laminated to another layer. The laminated packaging material is dried if necessary and accumulated in rolls. The rolls are kept in storage for a predetermined amount of time to allow the adhesive to cure before use in some applications.
Although there are many possible types of adhesives, polyurethane based adhesives are preferred for use in flexible packaging materials because of their many desirable properties including good adhesion to the materials in each layer, high peel strength, resistance to heat such as from heat sealing or retorting, and resistance to chemically aggressive products. Typically, an isocyanate-containing polyurethane prepolymer obtained by the reaction of excess diisocyanate with a polyether and/or polyester containing two or more active hydrogen groups per molecule is used in combination with a second component. The second component is usually a polyether polyol and/or a polyester polyol. The two components are combined just before use and in a predetermined ratio and applied on one of the film surfaces and the coated film is laminated to another substrate.
Solvent is used as a diluent for some polyurethane laminating adhesives as the viscosity of those adhesives is too high to apply them reliably in liquid form in a roll to roll laminating process. Solventless laminating adhesives (adhesives that can be applied at 100% solids and that do not contain either organic solvents or water) have a distinct advantage in that they can be applied and run at very high line speeds. This is due to the fact that no organic solvent or water has to be removed from the adhesive by drying. Solvent- or water-based laminating adhesives are limited to an application speed at which the solvent or water can be effectively dried in an oven. Typical line speeds for solvent-based and water-based laminating adhesives are 300 to 600 feet per minute due to the drying restrictions. Solventless adhesives, on the other hand, can be applied at 900 to even 2000 feet per minute, a line speed not possible with solvent-based and water-based laminating adhesives. Solventless laminating adhesives thus have a distinct advantage over solvent-based or water-borne adhesives.
In order that the proper coating weight of laminating adhesive is applied to the substrate, the adhesive must be “metered down” by transfer rolls to the application web or substrate. This is generally achieved by transferring the adhesive from a “puddle” between two rolls to a second and sometimes third or fourth roll before applying to the substrate. Each subsequent transfer rolls turn at a speed higher than the former roll so that there is less adhesive on each subsequent roll. Since these rolls are rotating at speeds up to 1000 rpm, incomplete transfer of the adhesive typically occurs with the formation of adhesive “droplets” that are released into the air around the metering rolls. These adhesive “droplets” are seen as aerosol droplets that are commonly called “adhesive mist”. Adhesive misting is undesirable.
Some solventless polyurethane laminating adhesives have to be heated to 100° C. to achieve a viscosity suitable for use in laminating packaging material. These high temperatures are difficult to achieve and control and are not energy efficient. In order to bring application temperature down, molecular weight of the polyurethane prepolymers was lowered, but the missing molecular weight had to be recovered by adding a second component to the adhesive mixture, mostly a blend of polyols. The application temperature for these modified polyurethane laminating adhesives could be reduced to about 40° C.
Another concern for flexible packaging material manufactured for the use as food packaging is government regulations. Government regulations require that the food packaging be safe when in contact with food. Unreacted isocyanate monomers can migrate into the food. This is problematic especially for aromatic isocyanate based adhesives. The reaction of these monomers with moisture in the packaged food turns them into primary aromatic amines, which are carcinogenic and not allowed in food items. One solution is to keep the flexible packaging material in storage until the adhesive components are fully reacted. After the adhesive components are fully reacted the flexible packaging material is formed into pouches. Unfortunately in case of laminating adhesives using lower molecular weight prepolymers and polyols this could take a long time, even up to a couple of weeks, and involve storage of large amounts of expensive laminating material before it can be used.
One strategy used in laminating adhesives to reduce cure time is to reduce isocyanate monomer content of the isocyanate prepolymers. This can be achieved by stripping monomers from the adhesive as described in EP 1 518 874. But this process is technically challenging, time consuming and expensive. Another strategy used in polyurethane laminating adhesives to reduce the monomer content is to utilize the different reactivity of the isocyanate groups of asymmetric multifunctional isocyanates. This approach is described in EP 0 150 444. The downside of this approach is that low monomer prepolymers cannot be manufactured with symmetrical isocyanates following this procedure. In both strategies, stripping and usage of asymmetric diisocyanates as raw materials, the viscosity of the resulting prepolymer is high as compared to the second generation systems. Nevertheless polyurethane adhesives based on these types of prepolymers reduce the storage time needed to be FDA compliant dramatically. But these systems require application at 70° C., which is not desirable for manufacturing or cleaning the laminating machinery.
Another concern of two component systems is the pot-life. Unless otherwise specifically described pot-life is the time required for the mixed adhesive to double its as mixed viscosity. For example, in a system that is applied at 40° C. with an as mixed viscosity of 1000 cps, the pot-life would be the time needed for that mixed adhesive to reach 2000 cps. Typically 2-component polyurethane adhesives have pot-lives of 15-20 minutes. In flexible packaging material lamination once the adhesive viscosity increases to a certain point the machinery must undesirably be shut down and cleaned. In order to maximize pot-life and minimize machinery shutdown and cleaning, special dosing units, so called meter mix dispense units (MMD's) are used to feed freshly mixed adhesive to the application station on an as needed basis.
As long as these systems are well maintained the isocyanate component and polyol component will be mixed in a specific mix ratio and the freshly mixed adhesive will (semi-)continuously be fed into the application station of the laminator. In case of an unpredicted interruption of the manufacturing process, for example due to a web break on the laminator, the mixed adhesive in the laminating machinery will build up viscosity and have to be discarded and the laminator applicator rolls will have to be cleaned. Fast curing adhesives with short pot lives tend to cause more problems in these cases and are not very desirable. Therefore a long pot-life is desired by laminating machine operators.
If the MMD is not well maintained the mix ratio can shift and the adhesive would be applied with too much polyol component or too much isocyanate component. The typical recommendation for a variance in the mix ratio is only ±5 wt %. If the mix ratio shifts and more than 5% excess polyol component is used in the mixture the cured adhesive will not show full performance, for example it will have lower heat resistance, leading to failures during heating or retorting of the sealed food package. If the mix ratio shifts and more than 5% excess isocyanate component is used the flexible packaging material can need an extended time to cure and reach FDA compliance, sometimes up to weeks.
A further concern of laminating adhesives is resistance to external solvents. The exterior surface of flexible packages is typically covered with printing or graphics. Flexographic printing inks used on flexible packages are blended with monoalcohols like Dowanol PM to improve the laydown of the ink. Depending on their vapor pressure these alcohols are easy or difficult to remove from the printed film. Higher molecular weight alcohols like Dowanol PM, which chemically is propyleneglycol mono-methylether, are referred to a “high boilers”. The solvents act as a polyurethane chain stopper and undesirably interfere with polymerization and bond strength of a polyurethane adhesive.
If “high boilers” are not removed completely they can undesirably interfere with the two component polyurethane adhesive reaction in the laminating flexible packaging material. To avoid undesirable effects the recommended limit for mono-alcohols in flexible laminating material applications is very low (<3,900 mg/ream of material, preferably <1,000 mg/ream of material and some applications require <100 mg/ream of material).
It would be desirable to provide a solvent less polyurethane laminating adhesive that does not require technically difficult preparation methods such as monomer stripping and which is useful with transfer roll lamination equipment and which can be applied at 100° C. or less and preferably about 40° C. or less.
It would also be desirable to provide a polyurethane laminating adhesive that has a long pot-life, for example greater than about 25 minutes, more preferably greater than about 30 minutes or more at application temperatures even when the isocyanate and polyol components are mixed off ratio with more than 5% excess, more preferably a 15% excess, more preferably a 25% excess, of polyol component or with more than 5% excess, more preferably a 15% excess, more preferably a 25% excess, of isocyanate component.
It would be desirable to provide a polyurethane laminating adhesive that can produce flexible packaging material with sufficient cured bond strength to withstand temperatures up to 100° C. and a boiling water environment even when the isocyanate and polyol components are mixed off ratio with more than 5% excess, more preferably a 15% excess, more preferably a 25% excess, of polyol component or with more than 5% excess, more preferably a 15% excess, more preferably a 25% excess, of isocyanate component.
It would be desirable to provide a polyurethane laminating adhesive that cures quickly and does not react with food simulants or products in a flexible package to form primary amines even when the isocyanate and polyol components are mixed off ratio with more than 5% excess, more preferably a 15% excess, more preferably a 25% excess, of polyol component or with more than 5% excess, more preferably a 15% excess, more preferably a 25% excess, of isocyanate component.